The present invention generally relates to laser projection technology and, more particularly, to calibration of laser projection systems that can project preprogrammed features at pre-defined locations and be used comparatively to verify the presence, absence or proper positioning of manufactured features, parts or subassemblies at these pre-defined locations.
Laser projection technology can provide a means to accurately project geometric patterns—such as squares, circles, crosses, and line segments—onto a surface, for example, that of a manufactured part. The technology can enable the precise location of a manufactured part relative to other manufactured parts, or some other convenient frame of reference, to be determined without using mechanical scales or other hard tooling.
The geometric patterns referred to above are typically projected onto a surface by “tracing” a laser beam, which when stationary may appear as a dot on the surface, over the surface quickly enough so that persistence of vision causes the laser beam dot on the surface to appear as a curve or line having a certain width or thickness, which generally is the diameter of the dot. The laser beam can be traced onto the surface using a pair of highly polished and flat mirrors that accurately rotate in a synchronic fashion to “bend” and direct the light beam. The mirrors can be computer controlled so that the patterns may be projected using CAD-generated data by tracing a laser beam onto the surface. For example, one laser projection system currently in use provides a beam with a nominal thickness or width of approximately 0.030 inches (in.) and a nominal positioning accuracy of ±0.015 in. at a nominal distance of 15 feet.
For general manufacturing use, and more specifically for practical use in the aircraft industry where the precision and accuracy of measurements need to be known, the nominal claims for beam width and positioning accuracy need to be verified. For example, a user may wish to verify the manufacturer's or vendor's claims to accuracy on first receiving a laser projection system. Of equal or greater importance is that the user may wish to check that various hazards and contingencies of the manufacturing environment have not contributed to a degradation of the accuracy of such a system, for example, by being bumped into or otherwise accidentally damaged. Routine quality control and maintenance checks may also be desired by the user of such a system. A question that arises is that since there are no physical attributes to the laser projection system except for the light that is perceived through the human eye, how can the user of the laser projection system be reasonably confident the beam is being projected accurately? In other words, can the “human factor” be taken out when verifying the beam accuracy? And consequently, is there a fixture that can be used to check the beam accuracy?
Previous methods for verifying beam accuracy have included using an artifact—such as an aluminum block—painted black with engraved shapes—such as lines, circles, and squares—having curved or linear elements that are the same nominal width as the laser beam located on the surface. The laser projection system is typically used to project the same shapes back onto the artifact to see if the laser beam-projected shapes align with the engraved shapes. The outcome of such a test then relies on a visual determination as to whether or not the laser beam “bleeds” over the engraved shapes, where, and to what extent. Therefore, a verification method has been sought that can place a lesser degree of reliance on visual acuity of the person performing the test and that can avoid the need for judgment and subjectivity on the part of the person performing the test.
As can be seen, there is a need for verification of laser beam width and positioning accuracy for laser beam projection systems. There is also a need for objective laser beam projection system accuracy verification—such as verification of laser beam width and positioning accuracy for laser beam projection systems—that avoids dependence on visual acuity and the need for judgment and subjectivity on the part of the person performing the test. There is also a need for a means for laser beam projection system testing that is portable and easily performed in a manufacturing environment.